Charles Arthur has written the best piece I’ve ever seen on the folly of putting too much stock into “market share” as a metric. Bookmark this one:

But if the market share figure is so useless, why does everyone quote it all the time?

Now we get to the key point. Because it’s easy to measure market share — much easier than measuring installed base, which requires large panels of people who you interview on a regular, repeated basis. (ComScore does this in the US, where it provides a picture of the installed base of smartphone users that is consistent back to the end of 2009. Its figures for the three months to September 2013 show a 51.8% installed base for Android — that’s 76.6m — and 40.6% for iPhone — that’s 60m. It’s not 80% Android; not even close.)

Plus “market share” gives journalists who like nothing better than a metaphorical horse race (look at the preponderance of polls, especially in the US presidential election) something to write about. Trouble is, it doesn’t necessarily give us useful information.